Method of measurement on a machine tool and corresponding machine tool apparatus

ABSTRACT

A method of scanning an object using an analogue probe mounted on a machine tool, so as to collect scanned measurement data along a nominal measurement line on the surface of the object, the analogue probe having a preferred measurement range. The method includes controlling the analogue probe and/or object to perform a scanning operation in accordance with a course of relative motion, the course of relative motion being configured such that, based on assumed properties of the surface of the object, the analogue probe will be caused to obtain data within its preferred measuring range, as well as cause the analogue probe to go outside its preferred measuring range, along the nominal measurement line on the surface of the object.

This invention relates to a method of measuring an artefact, and inparticular to a method of scanning an artefact using an analoguemeasurement tool mounted on a machine tool.

It is known to mount a measurement probe in a machine tool spindle, formovement with respect to a workpiece, in order to measure the workpiece.In practice, the probe has typically been a touch trigger probe, e.g. asdescribed in U.S. Pat. No. 4,153,998 (McMurtry), which produces atrigger signal when a stylus of the probe contacts the workpiecesurface. This trigger signal is taken to a so-called “skip” input of themachine tool's numeric controller (NC). In response, relative movementof the object and workpiece are stopped and the controller takes aninstantaneous reading of the machine's position (i.e. the position ofthe spindle and the probe relative to the machine). This is taken frommeasurement devices of the machine such as encoders which provideposition feedback information in a servo control loop for the machine'smovement. A disadvantage of using such a system is that the measurementprocess is relatively slow resulting in long measurement times if alarge number of measurement points are required.

Analogue measurement probes (also commonly known as scanning probes) arealso known. Contact analogue probes typically comprise a stylus forcontacting the workpiece surface, and transducers within the probe whichmeasure the deflection of the stylus relative to the probe body. Anexample is shown in U.S. Pat. No. 4,084,323 (McMurtry). In use, theanalogue probe is moved relative to the surface of the workpiece, sothat the stylus scans the surface and continuous readings are taken ofthe outputs of the probe transducers. Combining the probe deflectionoutput and the machine position output allows co-ordinate data to beobtained thereby allowing the position of the workpiece surface to befound at a very large number of points throughout the scan. Analogueprobes thus allow more detailed measurements of the form of theworkpiece surface to be acquired than is practically possible using atouch trigger probe.

As will be understood (and explained in more detail below in connectionwith FIG. 2), an analogue probe has a limited measurement range.Furthermore, the analogue probe might have a preferred measurementrange. The analogue probe might be able to obtain data outside itspreferred measurement range, but the data obtained outside this rangecould be less preferred, for instance because it could be considered tobe less accurate than the data obtained within the preferred measuringrange. The boundaries of the preferred measurement range can varydepending on many different factors, including the type of probe, thecalibration routine used, and even for instance the object beingmeasured. In many circumstances it can be preferred to ensure that theanalogue probe is kept within its preferred measurement range as itscans along the surface of the workpiece. The preferred measurementrange of an contact analogue probe can be for example +/−0.8 mm in anygiven dimension or smaller, for example in some circumstances as smallas +/−0.3 mm in any given dimension. (These values could be measuredfrom the stylus' rest position). Furthermore, the actual preferredmeasurement range could be even smaller than the figures given abovebecause it might be that a minimum amount of deflection could be neededto enter the preferred measuring range. Accordingly, although thepreferred measuring range might be +/−0.5 mm from the rest position, atleast the first +/−0.05 mm of deflection or for example the first +/−0.1mm of deflection might not be within the preferred measuring range(again, this is explained in more detail below in connection with FIG.2). Accordingly, as will be understood, real-time management of theprobe/workpiece positional relationship is required to avoid situationsin which the analogue probe falls outside its preferred measurementrange.

This is why analogue probes have typically only been used with dedicatedco-ordinate measuring machines (CMMs) even though analogue probes havebeen known per se for many years. This is because CMMs have dedicatedreal-time control loops to enable such management of probe deflection tooccur. In particular, in CMMs a controller is provided into which aprogram is loaded which defines a predetermined course of motion for themeasurement probe to move along relative to a workpiece. The controllergenerates motor control signals from the program which are used toactivate motors to induce movement of the measurement probe. Thecontroller also receives real-time position data from the machine'sencoders and also deflection data (in the case of a contact probe) fromthe analogue probe. In order to accommodate for variations in thematerial condition of the workpiece a dedicated control loop arrangementexists. This comprises a feedback module into which the above mentionedmotor control signals and deflection data are fed. The feedback modulesuses logic to continuously update (based on the deflection data) anoffset control vector which in turn is used to adjust the abovementioned motor control signal generated from the program before it issent to the CMM's motors so as to try to maintain probe deflectionwithin the preferred measuring range as the analogue probe scans theworkpiece. This all happens within a closed loop control loop with aresponse time of less than 1 to 2 ms. This is for example described inWO2006/115923.

Such tight control over probe positioning plus the ability to processreal-time stylus deflection data allows such dedicated CMMs to scancomplex articles that deviate from their expected shape and even to scanarticles of unknown shape.

To date, analogue probes have not been widely used for machine toolscanning applications. This is due to the inherent nature of manycommercially available machine tools which do not facilitate thereal-time control of the analogue probe that CMMs provide. This isbecause machine tools are primarily developed to machine workpieces andthe use of measurement probes on them to measure workpieces isessentially an after-thought. Machine tools are therefore typically notconfigured for real-time control using data from an analogue measurementprobe. Indeed, it is often the case that the machine tool has noin-built provision for the direct receipt of deflection data from themeasurement probe. Rather, the probe has to communicate (e.g.wirelessly) with an interface which receives the probe deflection dataand passes the data to a separate system which subsequently combines thedeflection data with machine position data so as to subsequently formcomplete object measurement data, for instance as described inWO2005/065884.

This makes it difficult to use an analogue probe on a machine tool toobtain scanned measurement data about known objects, because anyvariation from the expected shape of the object can cause the probe toover deflect and hence cause the measurement process to fail (whereas ona CMM the probe's course of motion could be updated quickly enough toensure that the probe doesn't over deflect). This also makes itdifficult to use an analogue probe on a machine tool to obtain scannedmeasurement data about unknown objects because this inherently requiresthe probe's course of motion to be updated quickly enough so as to avoidover deflection.

Techniques for overcoming the problems of using an analogue scanningprobe on a machine tool have been developed. For instance, drip feedtechniques are known in which the program instructions are loaded intothe machine tool's controller in a drip fed manner. In particular, eachinstruction causes the probe to move by a tiny distance (i.e. less thanthe probe's preferred deflection range), and the probe's output isanalysed to determine the extent of deflection, which in turn is used togenerate the next instruction to be fed into the controller. However,such a technique is still much more limited than the scanning techniquesthat can be performed using an analogue scanning probe on a CMM. Inparticular, such a method is very slow and inefficient.

WO2008/074989 describes a process for measuring a known object whichinvolves repeating a measurement operation according to an adjusted pathif a first measurement operation resulted in over or under deflection.

The problem can also be further compounded when using analogue probes onmachine tools because due to their construction (which enables them tobe able to used within the harsher environments that machine toolsprovide and the greater accelerations and forces they are exposed tosuch as when they are auto-changed into/out of a machine tool's spindle)they often have a much smaller measurement range than those analogueprobes which are for use with CMMs. This can therefore give even lessroom for error compared to analogue probes used on CMMs. For example, amachine tool analogue probe could have a measurement range of +/−0.8 mmin any given dimension or smaller (measured from the stylus' restposition), for example in some circumstances +/−0.5 mm in any givendimension or smaller, and for example in some circumstances no biggerthan +/−0.3 mm in any given dimension. This can therefore give even lessroom for error compared to analogue probes used on CMMs. As mentionedabove, a minimum deflection might also be required in order to enter thepreferred measuring range.

As a specific example, the measurement range could be defined by amaximum deflection 0.725 mm and a minimum deflection of 0.125 mm(measured from the stylus' rest position). Accordingly, in this case,this can mean that the surface can be +/−0.3 mm from nominal whilstmaintaining an accurate measurement. However, this figure can besmaller, and for instance it is known to for surface uncertainties to beas small as +/−0.1 mm, which corresponds to a maximum probe deflectionof around +/−0.325 mm and a minimum probe deflection of +/−0.125 mm.

According to a first aspect of the invention there is provided, a methodof scanning an object using an analogue probe mounted on a machine tool,so as to collect scanned measurement data along a nominal measurementline on the surface of the object, the analogue probe having a preferredmeasurement range, the method comprising: controlling the analogue probeand/or object to perform a scanning operation in accordance with acourse of relative motion, the course of relative motion beingconfigured such that the position of the preferred measuring rangerelative to the surface of the object is controlled in a manner that,based on assumed properties of the surface of the object, will cause theanalogue probe to obtain data within its preferred measuring range, aswell as cause the analogue probe to be outside its preferred measuringrange, along the nominal measurement line on the surface of the object.

Accordingly, rather than trying to always keep the analogue probe withinits preferred measurement range, the present invention therefore workson the basis that it expects the analogue probe to move within andoutside its preferred measurement range along the nominal measurementline on the surface of the object. This could be such that it expectsthe analogue probe to obtain measurements both within and outside itspreferred measuring range along the nominal measurement line on thesurface of the object. Indeed, the method can be configured such thatthe position of the analogue probe's preferred measurement range withrespect to the surface of the object is controlled such that theanalogue probe is caused to deliberately obtain measurements both withinand outside its preferred measurement range, along the nominalmeasurement line on the surface of the object. This can improve theefficiency by which object measurement data is obtained using ananalogue probe on a machine tool.

It could be that the course of relative motion is configured such thatthe position of the preferred measuring range relative to the surface ofthe object is controlled in a manner that, based on assumed propertiesof the surface of the object, will cause the analogue probe to obtaindata within its preferred measuring range, as well as cause the analogueprobe to exceed its preferred measuring range, along the nominalmeasurement line on the surface of the object. It could be that thecourse of relative motion is configured such that the position of thepreferred measuring range relative to the surface of the object iscontrolled in a manner that, based on assumed properties of the surfaceof the object, will cause the analogue probe to obtain data within itspreferred measuring range, as well as cause the analogue probe to exceedand or fall-short of its preferred measuring range, along the nominalmeasurement line on the surface of the object.

The method could be configured such that the course of relative motionis configured such that during the scanning operation, based on assumedproperties of the surface of the object, the position of the preferredmeasuring range relative to the surface of the object in a directionnormal to the surface of the object (e.g. the height) varies along thenominal measurement line.

The method can further comprise filtering the data obtained from theanalogue probe so as to obtain select scanned measurement data. Themethod can comprise filtering the data obtained from the analogue probeso as to obtain data relating to scanned measurement data obtainedpredominantly from either within or from outside the analogue probe'spreferred measurement range. The method can comprise filtering the dataobtained from the analogue probe so as to obtain select scannedmeasurement data relating predominantly to scanned measurement dataobtained from within the analogue probe's preferred measurement range.The method can comprise filtering the data obtained from the analogueprobe so as to obtain select scanned measurement data relatingsubstantially to only scanned measurement data obtained from within theanalogue probe's preferred measurement range.

The method can comprise collating said filtered data into a further dataset. Accordingly, for instance, the further data set could comprisescanned measurement data relating to the surface of the object that wasobtained within the analogue probe's preferred measurement range. Thefurther data set could be output as measurement data of the object.

Accordingly, the method can comprise collecting and outputting scannedmeasurement data obtained within the analogue probe's preferredmeasuring range as the measurement of the object. In line with theabove, such collecting and outputting can comprise filtering the dataobtained from the analogue probe so as to obtain, and provide as themeasurement of the object, select object measurement data obtained fromwithin the analogue probe's preferred measurement range.

The preferred measuring range can be less than the total measuring rangeof the analogue probe. In the case of a contact probe, the preferredmeasuring range can be less than the total deflection range of theanalogue probe. Accordingly, the preferred measuring range could be asubset of the analogue probe's entire measurement range. As mentionedabove, the exact boundaries of the preferred measurement range can varyfrom probe to probe and even from measurement operation to measurementoperation for any given probe. It could be the range for which theanalogue probe has been calibrated for any given measurement operation,e.g. to give a desired level of accuracy.

The method can comprise generating and executing (e.g. as part of asecond scanning operation) a new course of relative movement of theanalogue probe and object based on the measurement data obtained duringthe previous scanning operation. The new course of relative movement cancomprise the analogue probe traversing substantially the same line ofmeasurement across the surface of the object. However, in this case therelative movement can be controlled such that the relative position ofthe analogue probe and object is such that the analogue probe obtainsmeasurements within its preferred measurement range for a greaterproportion of the measurement path than for the previous measurement ofthe object. In particular, the new path of relative movement for theanalogue probe and object to follow can be configured such that theanalogue probe obtains measurement data within its preferred measurementrange along substantially the entire length of the same nominal line.

The object and analogue probe could be configured to move relative toeach other along a predetermined path of relative motion so that theanalogue probe obtains scanned measurement data along the nominalmeasurement line on the surface of the object.

The predetermined path of relative motion can be configured such thatthe analogue probe proceeds in a manner that, based on assumedproperties of the surface of the object, causes the position of theanalogue probe's preferred measurement range to repeatedly rise and fallrelative to the surface of the object as it moves along the nominalmeasurement line. Accordingly, this could be so as to cause, based onassumed properties of the surface of the object, the analogue probe tooscillate between obtaining data within and outside its preferredmeasuring range (e.g. under and within, or within and beyond, or under,within and beyond preferred measuring range) along the nominalmeasurement line. For instance, the predetermined path of relativemotion can be configured such that the analogue probe moves in anundulating, sinusoidal or wavy manner as it moves along the nominalmeasurement line.

The predetermined path of relative motion can be configured such that,based on assumed properties of the surface of the object, the analogueprobe is maintained in a position sensing relationship with the surfaceof the object as it is moved along the nominal measurement line. Thiscould particularly be the case with the above described embodiment inwhich the predetermined path of relative motion is configured such thatthe analogue probe proceeds in a manner that, based on assumedproperties of the surface of the object, causes the position of theanalogue probe's preferred measurement range to repeatedly rise and fallrelative to the surface of the object as it moves along the nominalmeasurement line.

The course of relative motion could be configured such that during thescanning operation the analogue probe's preferred measuring rangetraverses across the object along the nominal measurement line aplurality of times. The positional relationship of the analogue probeand object can be different for different traverses. The analogue probecan obtain measurement data within different regions of its entiremeasuring range for different traverses. The form of the route theanalogue probe and object take relative to each other can besubstantially the same for each traverse such that the object ismeasured along the nominal measurement line on the surface of the objecta plurality of times. However, the position of the analogue probe andobject could be offset relative to each other for different traverses.

Accordingly, the course of motion could be configured such that fordifferent traverses the analogue probe obtains measurement data withinits preferred measurement range for different parts of the object, alongthe same nominal measurement line on the surface of the object. The formof the route that the preferred measuring range takes relative to thesurface can be substantially the same for successive traverses.Accordingly, the height of the route from the surface at at least onepoint along the nominal measurement line (and preferably along theentire length of the nominal measurement line) can be different fordifferent traverses. In particular, the traverses can be offset fromeach other, such that for different traverses the analogue probe obtainsmeasurement data within its preferred measurement range for differentparts of the object, along the same nominal measurement line on thesurface of the object. In other words, the course of motion could beconfigured such that the preferred measuring range traverses across theobject along the nominal measurement line, at least twice, each traversebeing substantially parallel to each other but at different nominalheights to the surface of the object. The nominal height could increaseover successive traverses. Preferably, the nominal height decreases oversuccessive traverses.

Accordingly, the course of motion could be configured such that at leasta first and second traverses are performed, and in which during thesecond traverse the analogue probe obtains measurement data within itspreferred measurement range for at least a part of the object for whichdata was obtained outside of the probe's preferred measurement rangeduring the first traverse.

As mentioned above, the position of the analogue probe's preferredmeasurement range above the surface of the object can be different fordifferent traverses. The position of the analogue probe's preferredmeasurement range relative to the surface of the object could rise oversuccessive traverses. Preferably, the position of the analogue probe'spreferred measurement range relative to the surface of the object fallsover successive traverses. The position could be measured between areference point with respect to the preferred measuring range (e.g. apoint within the preferred measuring range, such as the mid-point of thepreferred measuring range) and the surface of the object (e.g. thenominal surface of the object). Accordingly, for instance, preferablythe line along which the centre of the preferred measuring range followsfor each pass could, on average, progressively fall (e.g. get closerto/penetrate deeper into) with respect to the surface of the object oversuccessive traverses. This could happen in a step-by-step manner, e.g.at the end of each traverse.

The course of relative motion can be configured such that the differencebetween previous and subsequent traverses is sufficiently small suchthat if along the previous traverse no surface measurement data wasobtained, the subsequent traverse will not cause the analogue probe toobtain object surface measurement data that exceeds its entiremeasurement range, and for example will not cause the analogue probe toobtain data beyond its preferred measuring range. Optionally, traversesare offset from each other in steps that are no bigger than, and forinstance are smaller than, the entire measurement range of the probe.For example, traverses can be offset from each other in steps that areno bigger than, and for instance are smaller than, the preferredmeasuring range of the probe.

Surface measurement data obtained within the preferred measuring rangefrom different traverses can be collated so as to form a measurementdata set which represents the surface of the object along the nominalmeasurement line. As mentioned above, the course of motion can beconfigured such that for different traverses the analogue probe obtainsmeasurement data within its preferred measurement range for differentparts of the object, along the same nominal measurement line on thesurface of the object. Preferably, the course of motion is configuredsuch that portions of the surface for which measurement data is obtainedwithin the preferred measurement range overlap between successivepasses. In this case, the measurement data set could represent acontinuous length of the surface along the nominal measurement line, andpreferably represent the surface along the entire length of the nominalmeasurement line. However, it might be that the portions do not overlapwhich could therefore mean that might be gaps in the measurement dataset.

The nominal surface shape of the object might not be known. The nominalsurface shape of the object could be known. In this case the shape ofthe measurement path across the object can be configured to besubstantially parallel to nominal surface shape. That is the path acrossthe object the preferred measurement range is configured to take can beconfigured to be substantially parallel to the nominal surface shape.

The analogue probe could be a non-contact analogue probe, for instancean optical, capacitance or inductance probe. In this case, the preferredmeasurement range could be a distance or separation range between a partof the analogue probe (e.g. the workpiece sensing part) and theworkpiece surface. Accordingly, the preferred measurement range couldcomprise upper and lower boundaries or thresholds relating to maximumand minimum probe-object separations. The analogue probe can be acontact analogue probe. For instance, the analogue probe could be acontact analogue probe with a deflectable stylus for contacting theobject. In this case, the preferred measurement range can be a preferredstylus deflection range. Accordingly, the preferred measurement rangecould comprise upper and lower boundaries or thresholds relating tomaximum and minimum stylus deflections.

The object could be an object that was (and/or is to be) machined on themachine on which the analogue probe is mounted. Accordingly, the methodcould comprise, the same machine tool machining the object, for exampleprior to the above described measuring steps. Optionally machining couldtake place after the above described measuring steps. Suchpost-measurement machining could take place on the same machine tool onwhich the measurement occurred. Such post-measurement machining could bebased on measurement data obtained during the above describedmeasurement steps. The machine tool could be a cutting machine, such asa metal cutting machine.

The analogue probe could be a sealed analogue probe. That is theanalogue probe could be sealed so as to protect internal sensorcomponentry from external contaminants. For instance, the probe couldcomprise a probe body which houses a sensor for either directly orindirectly measuring the surface of an object, in which the sensor issealed from external contaminant. For instance, in the case of a contactprobe, the probe could comprise a probe body, a stylus member and asensor for measuring displacement of the stylus member relative to thehousing, in which at least a first compliant sealing member is providedwhich extends between the probe body and relatively moveable stylusmember, such that the sensor is contained within a sealed chamber andthereby sealed from external contaminants.

The object can be a blade. For instance, the blade could be a blade of aturbine engine.

Accordingly, this application describes a method of scanning an objectusing an analogue probe mounted on a machine tool, the analogue probehaving a preferred measurement range, the method comprising: performinga scanning measurement operation which comprises moving the object andanalogue probe relative to each other so that the analogue probe obtainsscanned measurement data along a nominal measurement line on the surfaceof the object, in which some of the data obtained during the scanningmeasurement operation along the nominal measurement line is within theanalogue probe's preferred measurement range and some is outside theprobe's preferred measurement range.

According to a second aspect of the invention there is provided acomputer program comprising instructions which when executed by amachine tool apparatus causes the machine tool apparatus to perform theabove described method.

According to a third aspect of the invention there is provided acomputer readable medium comprising instructions which when executed bya machine tool apparatus causes the machine tool apparatus to performthe above described method.

According to a fourth aspect of the invention there is provided amachine tool apparatus comprising a machine tool, an analogue probemounted on the machine tool, and a controller configured to control therelative movement of the analogue probe and an object to be measured soas to so as to collect scanned measurement data along a nominalmeasurement line on the surface of the object, and in particular so asto control the analogue probe and/or object in accordance with a courseof relative motion such that the position of the preferred measuringrange relative to the surface of the object is controlled in a mannerthat, based on assumed properties of the surface of the object, willcause the analogue probe to obtain data within its preferred measuringrange, as well as to exceed its preferred measuring range, along thenominal measurement line on the surface of the object.

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying drawings in which:

FIG. 1 is a schematic diagram showing the system architecture for amachine tool;

FIGS. 2( a) to (c) are schematic diagrams illustrating the measurementrange of analogue measurement probes;

FIG. 3 is a system flow chart illustrating the flow of control during ameasurement operation according to an embodiment of the invention;

FIG. 4 schematically illustrates the nominal path of a stylus tipaccording to a first embodiment of the invention;

FIGS. 5( a) and 5(b) schematically illustrate side and isometric viewsof a nominal path of a stylus tip according to a second embodiment ofthe invention;

FIGS. 6( a) and 6(b) respectively illustrate nominal path of a stylustip according to a third and fourth embodiments of the invention; and

FIG. 7 illustrates nominal paths of a stylus tip according to apreliminary scan and a subsequent scan generated based on data obtainedduring the preliminary scan, according to a further embodiment of theinvention.

Referring to FIG. 1, there is shown a machine tool apparatus 2comprising a machine tool 4, a controller 6, a PC 8 and atransmitter/receiver interface 10. The machine tool 4 comprises motors(not shown) for moving a spindle 12 which holds an analogue probe 14relative to a workpiece 16 located on a table 15. The location of thespindle 12 (and hence the analogue probe 14) is accurately measured in aknown manner using encoders or the like. Such measurements providespindle position data defined in the machine co-ordinate system (x, y,z). A numerical controller (NC) 18 (which is part of the controller 6)controls x, y, z movement of the spindle 12 within the work area of themachine tool and also received data relating to the spindle position.

As will be understood, in alternative embodiments relative movement inany or all of the x, y and z dimensions could be provided by movement ofthe table 15 relative to the spindle. Furthermore, relative rotationalmovement of the analogue probe 14 and workpiece 16 could be provided bya part of the spindle 12 (e.g. a rotating/articulated head mounted onthe spindle) and/or a part of table 15 (e.g. a rotary table).Furthermore, movement might be restricted to fewer dimensions, e.g. onlyx, and/or y. Further still, the embodiment described comprises acartesian machine tool, whereas will be understood this need notnecessarily be the case and could be instance be a non-cartesian machinetool. Further still, many other different types of machine tools,including lathes, and parallel-kinematic machines, and robot arms areknown and could be used with the invention.

In the embodiment described, the analogue probe 14 is a contact analogueprobe which comprises a probe body 20, a workpiece contacting stylus 22extending from the probe body 20, and has a surface detection region inthe form of a workpiece contacting tip 24 (which in this case in theform of a spherical stylus ball) at the distal end of the stylus 22. Theanalogue probe 14 measures deflection of the stylus 22 in a probegeometry system (a, b, c). (However, as will be understood, this neednot necessarily be the case, and for instance the analogue probe couldmeasure deflection in only 1 or 2 dimensions, or even provide an outputindicative of the extent of deflection, without any indication of thedirection of deflection). The probe 14 also comprises atransmitter/receiver (not shown) that wirelessly communicates with thetransmitter/receiver interface 10 (e.g. via a radio, optical or otherwireless transmission mechanism).

As mentioned above, analogue measurement probes have a limitedmeasurement range. For instance with regard to contact analogue probes,they can have a physical maximum amount by which they can be deflectedin the x, y and z dimensions. Not only this, but it can be that theprobe is configured such that it works optimally within a certainsub-range of the maximum physical range. For instance, FIG. 2( a)illustrates the analogue probe of FIG. 1, and the solid line representsthe position of the stylus 22 at a rest (e.g. undeflected) position. Theoutermost stylus positions shown in dashed lines represent the maximumphysical deflection of the stylus in the x-dimension. However, it couldbe that the probe is configured such that it is most accurate when thestylus is deflected by an amount less than the maximum physicaldeflection. It could also be that the probe is configured such that itis most accurate when the stylus is deflected by a minimum lowerthreshold. For instance, the analogue probe 14 could have a preferredmeasurement range, the upper and lower boundaries of which are shown bystylus positions shown in FIG. 2( a) as dotted lines. Accordingly, ascan be seen there is a dead space ‘d’ (in the x-dimension) in the middleclose to the stylus' rest position which is outside the preferredmeasuring range.

As will be understood, the same will also be the case with deflection inthe y-dimension. Furthermore, in the described embodiment there is alsoa maximum physical deflection range in the z-axis as well as a sub-rangeof z-axis deflections (a preferred measurement range) within which theprobe is configured to provide the most accurate results.

The dotted line 28 shown in FIG. 2( b) schematically illustrates thescope of the analogue probe's 14 preferred measurement range taken inthe x and z dimensions. As will be understood, such a range actuallyextends in a three dimensions, and hence is actually approximately theshape of a squashed hemisphere with a small hole cut out in the middle.

The dotted lines of FIG. 2( c) also schematically illustrate thepreferred measurement range for a non-contact probe, such as aninductance probe. The inner and outer dotted lines represent the minimumand maximum probe/workpiece separation boundaries for optimum measuringperformance. As will be understood, the preferred measuring range shownfor the non-contact probe could be the entire measuring range or only asubset of the entire measuring range for the probe. As will beunderstood, the entire measuring range could be considered to be whatcan be referred to as the non-contact probe's surface detecting region.

As will be understood, the size of the preferred measuring range willvary from probe to probe. For a contact analogue probe, it could be forexample not more than +/−0.8 mm in any given dimension, for example notmore than +/−0.725 mm in any given dimension, for instance not more than+/−0.5 mm in any given dimension, for example in some circumstances notmore than +/−0.3 mm in any given dimension (taken from the stylus restposition). Of course, there might also be a dead-zone immediately aroundthe stylus position through which the stylus has to be deflected beyondbefore it enters the preferred measuring range, which could be forexample not less than +/−0.2 mm in any given dimension from the stylusrest position, for instance not less than +/−0.1 mm in any givendimension from the stylus rest position, e.g. not less than +/−0.125 mmin any given dimension (again, measured from the stylus rest position).

As described above, the present invention departs from the traditionalview that the probe must be maintained such that along the nominalmeasurement line on the surface of the object the probe always collectsdata within its preferred measurement range. Rather, as is clear fromthe embodiments described below, the invention enables measurementsalong the nominal measurement line to be obtained both within andoutside the probe's preferred measurement range and then subsequentlyfiltered as required.

FIG. 3 illustrates the general procedure 100 involved according to oneembodiment of the invention. The method starts at step 102 at whichpoint a model of the part to be measured is loaded into the PC 8. Asexplained in more detail below, this step need not necessarily beperformed in embodiments in which the workpiece to be measured isunknown. At step 104 a program defining a course of motion for theanalogue probe 14 to obtain scanned measurement data of the workpiece 16is generated. In the embodiment described, the course of motion isconfigured such that the analogue probe will obtain measurement databoth within and outside its preferred measurement range along a nominalmeasurement line on the surface of the object. As will be understood, inembodiments in which the workpiece 16 can be moved as well as, orinstead of the analogue probe 14 (e.g. by virtue of a movable table 15),then the program can also define a course of motion of the workpiece 16.In other words, step 104 comprises planning the relative course ofmotion between the analogue probe 14 and the workpiece 16 so that theanalogue probe 14 can collect scanned measurement data regarding theworkpiece 16. At step 106 the program is loaded into the NC 18 via theAPI 26. Step 108 involves performing the measurement operation andrecording measurement data. In particular, performing the measurementoperation comprises the NC 18 interpreting the program's instructionsand generating motor control signals which are used to instruct themachine tool's 4 motors (not shown) so as to move the analogue probe 14in accordance with the predetermined course of motion. Recordingmeasurement data comprises a number of procedures. In particular,spindle position data (x, y, z) (which as mentioned above is provided byencoders on the machine tool 4) is passed to the PC 8 via the NC 18.Furthermore, probe deflection data (a, b, c) (which as mentioned aboveis obtained by the analogue probe) is also passed to the PC 8 via theprobe transmitter/receiver interface 10. The PC 8 combines the spindleposition data (x, y, z) and the probe deflection data (a, b, c) toprovide a set of measurements that define the position of a surfacewithin the machine co-ordinate geometry.

Step 110 comprises the PC 8 filtering the recorded measurement data. Inthe particular embodiment described, this comprises the PC 8 filteringthe recorded measurement data for measurement data that was obtainedwithin the analogue probe's preferred measurement range. As will beunderstood, the data could be filtered in other ways, for instance, formeasurement data that was obtained outside the analogue probe'spreferred measurement range. As will be clear from the differentembodiments described above, how the filtering is performed, and the endresult obtained varies from embodiment to embodiment.

For instance (and as explained in more detail below), FIG. 4 illustratesa technique according to the invention for measuring a known part inwhich the stylus tip is moved across the workpiece 16 in an undulatingmanner so as to collect data within and outside its preferredmeasurement range. FIG. 5 illustrates a technique for measuring anunknown part by traversing back and forth across the same nominalmeasurement line on the surface of the part at different nominal stylustip positions, and FIG. 6 illustrates a similar technique but which isused for measuring a known part.

Referring first to FIG. 4, this illustrates that the stylus tip 24 couldbe configured such that its gross movement (illustrated by feint dottedline 30) is generally parallel to surface 17 of the workpiece 16.However, as illustrated by the dark dotted line 32, the relative courseof motion of the analogue probe and workpiece 16 is configured such thatthe nominal stylus tip centre point 23 is caused to undulate toward andaway from the surface as it travels along a nominal measurement line 19on the surface 17 of the object 16. The dotted circles 24A, 24B, 24Crepresent the nominal position of the stylus tip at three differentpoints along the nominal measurement line on the surface of the object16. As will be understood, these stylus tip positions 24A, 24B, 24C arenominal in that this is where the position the tip would be in at thosepoints along the nominal measurement line if the object was not there.The nominal measurement line 19 (which is more easily seen in FIG. 5( b)which illustrates a different embodiment of the invention) is the lineon the surface 17 of the object on which measurement data is to begathered. The line is nominal in that this is the expected line ofmeasurement on the object. As will be understood, the actual line ofmeasurement may be different if the location and/or material conditionof the object 16 is different to what is expected.

Such undulation of the nominal stylus tip centre point 23 could beachieved for instance by varying the probe's stand-off distance relativeto the surface 17 of the workpiece 16 as it traverses across theworkpiece 16. Optionally, if the probe were mounted on an articulatedhead, then this could be achieved by varying the angular position of theprobe about at least one of the head's rotational axes.

In the embodiment described, the course of motion is configured suchthat the nominal undulating motion 32 is configured such that for aperfect workpiece 16 (i.e. in which the actual workpiece correspondsexactly to the model workpiece) the probe's 14 stylus 22 is configuredto oscillate between being over and under deflected as it travels alongthe surface 17 of the workpiece 16, in between which the analogue probe14 collects measurement data within its preferred measurement range. Forinstance as shown in FIG. 4 at nominal stylus tip position 24A, thestylus deflection is such that the probe 14 obtains measurement datawithin is preferred measurement range whereas nominal stylus tippositions 24B and 24C the stylus is respectively under and overdeflected. Accordingly, it can be seen from the embodiment of FIG. 4that as the stylus tip 24 travels across the surface only selectportions (illustrated by the dashed and dotted segments 34) where thestylus deflection is within its preferred deflection range and henceonly select portions of the data obtained from the measurement probewill be obtained within its preferred measurement range.

As will be understood, the amplitude A of undulating position of thestylus centre tip is greatly exaggerated in FIG. 4 so as to aidillustration. As will be understood, the extent of the amplitude A willvary depending on many factors including the extent of the preferredmeasurement range, the extent the actual physical range of a deflectablestylus, the nominal workpiece dimensions and the expected level ofvariation in surface position. Nevertheless, by way of example only, theamplitude A can be as less than 5 mm, for example less than 2 mm, andfor example greater than 0.5 mm, for example 1 mm. Furthermore, thepitch P of the undulating motion will vary depending on many factors,such as those mentioned above and for example the density ofmeasurements required, and for example can be less than 100 mm and forexample greater than 10 mm, for example 50 mm.

Although the method of FIG. 4 results in only some of the measurementdata being obtained within the probe's preferred measurement range, thismethod of measuring can help to ensure that at least some measurementdata is obtained within the preferred measurement range despite thematerial condition of the workpiece 16 being different to what isexpected. For instance, with reference to FIG. 4, if the actual positionof the workpiece 16 is slightly offset such that its surface 17 islocated a fraction closer to the nominal probe tip centre line 32, thenmeasurement data will still be obtained, but rather than being obtainedat the points illustrated by dashed and dotted portions 34, thepreferred measurement data will be obtained at the peaks of the nominalprobe tip centre line 32.

FIGS. 5( a) and (b) illustrate an alternative embodiment in which thepart to be measured is unknown. In this case, no model of the part isloaded into the PC at step 102, and step 104 comprises generating astandard course of motion which can be used to obtain measurement dataabout the unknown part. The part could be unknown in that its shape anddimensions of at least one feature are unknown and are to be determined.In the embodiment described, the predetermined course of motion isconfigured such that the nominal probe tip centre 23 moves back andforth along the same nominal measurement line 19 on the surface 17 ofthe object 16, but at different nominal distances from the surface 17,as illustrated by dotted line 40. In the embodiment shown, each traverseacross the surface of the object is generally in a straight line, andalso constrained within one plane, however as will be understood thisneed not necessarily be the case. Indeed, for example, each traversecould involve causing the nominal stylus tip centre point to undulatemuch like that shown in FIG. 4. Furthermore, the path of each traversecould meander in a sideways direction, e.g. in a side-to-side motion.Furthermore, the predetermined course of motion need not necessarilymove the nominal probe tip centre in a back and forth manner. Forexample, each traverse could take place in the same direction.Furthermore, each traverse could, for example, comprise moving thenominal probe tip centre in a winding (e.g. spiral) manner across thesurface of the object.

As shown, the position of the analogue probe's preferred measuring rangewill fall relative to the surface 17 of the object 16 over successivetraverses. In particular its average position along the nominalmeasurement line above the surface falls over successive traverses. Inthe embodiment described, on the first traverse, the stylus tip 24 doesnot deflect enough to enter its preferred deflection range and hence nodata is obtained within the preferred measurement range. On the secondtraverse, the crest of the surface 17 causes the stylus tip 24 todeflect within its preferred deflection range and so measurement data isobtained within the probe's preferred measurement range for a portion ofthe pass, illustrated by the dashed and dotted portion 42. As can beseen for the third and forth traverses, again the stylus is deflectedwithin its preferred measurement range so as to obtain measurement datawithin the probe's preferred measurement range for portions 42 of thepasses. During step 110, these portions 42 of measurement data that havebeen obtained within the preferred measurement range can be filteredfrom the entire measurement data set and collated so as to provide a newset of measurement data regarding the object, all of which was obtainedwithin the analogue probe's 14 preferred measurement range. In theembodiment shown, the nominal course of motion of the probe tip centre23 is such that the portions 42 of data obtained within the preferredmeasurement range overlap between successive passes. However, this neednot necessarily be the case, which would therefore mean that there couldbe gaps in any final data set that is created from data obtained withinthe analogue probe's 14 preferred measurement range. Furthermore, aswith FIG. 4, the step toward the workpiece between each pass isexaggerated to aid illustration. The actual size of the step variesdepending on a number of factors, including the measuring range of theprobe but typically could for instance be as small as 0.2 mm and aslarge as 0.8 mm. Furthermore, although it is shown that the nominalstylus tip centre point steps toward the object after each traverse,this need not necessarily be the case. For instance, the nominal stylustip centre point could nominally get progressively closer along thelength of the pass such that it gradually approaches the object alongeach traverse.

In the embodiment described in connection with FIG. 5, the analogueprobe's stylus deflects beyond its preferred measurement range, butnever deflects beyond its maximum deflection threshold. However, inother embodiments it might be that the shape and dimensions of theobject, and/or the predetermined path of relative movement, is such thatthe analogue probe is at risk of its stylus deflecting beyond itsmaximum deflection threshold. In this case, during a scanning operation,the analogue probe's output can be monitored to check for such asituation and take corrective action. Such corrective action could be tohalt and abort the scanning operation. Optionally, such correctiveaction could be to adjust the predetermined path of relative movement sothat deflection of the stylus beyond its maximum deflection threshold isavoided. For instance, at the end of each traverse, it could bedetermined if on the next, or a future, traverse the stylus is likely todeflect beyond its maximum deflection threshold, and if so then adjustthe predetermined path of relative movement.

Such a rastering approach adopted described for unknown parts inconnection with FIG. 5 can also be useful even if the nominal shape ofthe workpiece is known. For instance, with reference to FIG. 6( a) thereis shown a situation in which the actual surface shape 17 of theworkpiece 16 deviates from its nominal surface shape 17′ in that it hasan unexpected dip 27. Accordingly, if the stylus tip 24 were to follow apath 50 substantially parallel to the expected nominal surface shape 17,then it would result in no measurement data being obtained within theanalogue probe's preferred measurement range for the dipped part of thesurface. However, as illustrated in FIG. 6( b) using a path 52 whichadopts the rastering approach enables the measurement data obtainedalong the path 52 that was obtained within the analogue probe'spreferred measurement range (such data being illustrated by dash and dotportions 54) to be filtered and collated, so as to thereby providemeasurement data that was obtained within the analogue probe's preferredmeasurement range for the entirety of the nominal measurement line 19for the actual surface shape 17.

As will be understood, the filtering could be achieved in many differentways. For instance, it could be done at source, in that only dataobtained within the probe's preferred measurement range is reported bythe analogue probe and/or receiver/transmitter interface 10. Optionally,all the data from the analogue probe is reported, by only thosemeasurements which were obtained within the analogue probe's preferredmeasurement range are combined with spindle (i.e. analogue probe)position data. In an alternative embodiment, all analogue probe data iscombined with spindle position data, and then the combined data issubsequently filtered to remove the combined data which containsanalogue probe data outside the preferred measurement range.

The above described embodiments filter for and collate data that wasobtained within the analogue probe's preferred measurement range. Aswill be understood, this need not necessarily be the case and insteadfor instance a method according to the invention could filter for andcollate data outside the preferred measurement range, or indeed onlyreport data that it outside the preferred measurement range. This mightbe useful for instance when it is only important to know when a part isout of tolerance (and possibly for example by how much).

In the embodiments described above, the path along which the analogueprobe and object are moved relative to each other is predetermined. Inparticular, the entire path is determined before the scanning operationis begun. However, this need not necessarily be the case. For instance,with respect to the embodiments described in connection with FIGS. 5 and6, the path of relative movement could be generated on atraverse-by-traverse basis. For example, a first traverse along thenominal measurement line could be completed, and if it is determinedthat not all the measurement data obtained along the nominal measurementline was within the analogue probe's preferred measurement range, then asubsequent traverse could be performed in which the position of theprobe's preferred measurement range is at a different position withrespect to the object along the traverse along the nominal measurementline. This process could be continually repeated until measurement datawithin the analogue probe's preferred measurement range has beenobtained along the entire length of the nominal measurement line.

In other embodiments, the method of the invention can comprisegenerating and executing (e.g. as part of a second scanning operation) anew course of relative movement of the analogue probe and object basedon the measurement data obtained during the previous scanning operation(e.g. during a scanning operation according to the embodiments of FIGS.4, 5 and 6). The new course of relative movement can comprise theanalogue probe traversing substantially the same line of measurementacross the surface of the object. However, in this case the relativemovement can be controlled such that the relative position of theanalogue probe and object is such that the analogue probe obtainsmeasurements within its preferred measurement range for a greaterproportion of the measurement path than for the previous measurement ofthe object. In particular, the new path of relative movement for theanalogue probe and object to follow can be configured such that theanalogue probe obtains measurement data within its preferred measurementrange along substantially the entire length of the same nominal line.

This is the case shown in FIG. 7, which is a replication of FIG. 6( b),except that solid line 56 illustrates the path of the probe tip centrewhich was generated from the data obtained during the scan illustratedby the dotted line 52.

The above described scanning operations can be performed at high speed(e.g. with the workpiece sensing part (e.g. the stylus tip 24) andobject travelling relative to each other at least at 16 mm/s, preferablyat least at 25 mm/s, more preferably at least at 50 mm/s, especiallypreferably at least at 100 mm/s, for example at least at 250 mm/s)because it doesn't matter whether the probe 14 obtains data outside itspreferred measuring range.

1. A method of scanning an object using a contact analogue probecomprising a deflectable stylus for contacting the object mounted on amachine tool, so as to collect scanned measurement data along a nominalmeasurement line on the surface of the object, the analogue probe havinga preferred measurement range, the method comprising: controlling thecontact analogue probe and/or object to perform a scanning operation inaccordance with a course of relative motion, the course of relativemotion being configured such that, based on assumed properties of thesurface of the object, the analogue probe will be caused to obtain datawithin its preferred measuring range, as well as cause the analogueprobe to go outside its preferred measuring range, along the nominalmeasurement line on the surface of the object.
 2. A method as claimed inclaim 1, comprising collecting and outputting scanned measurement dataobtained within the contact analogue probe's preferred measuring rangeas the measurement of the object.
 3. A method as claimed in claim 2, inwhich said collecting and outputting comprises filtering the dataobtained from the contact analogue probe so as to obtain, and provide asthe measurement of the object, select object measurement data obtainedfrom within the contact analogue probe's preferred measurement range. 4.A method as claimed in claim 1, in which the contact analogue probeproceeds in a manner that, based on assumed properties of the surface ofthe object, causes the position of the contact analogue probe'spreferred measurement range to repeatedly rise and fall relative to thesurface of the object as it moves along the nominal measurement line. 5.A method as claimed in claim 1, in which the contact analogue probe ismaintained in a position sensing relationship with the surface of theobject as it is moved so as to collect data along the nominalmeasurement line.
 6. A method as claimed in claim 1, in which the courseof relative motion is configured such that during the scanning operationthe contact analogue probe's preferred measuring range traverses acrossthe object along the nominal measurement line a plurality of times.
 7. Amethod as claimed in claim 6, in which for different traverses thecontact analogue probe obtains measurement data within its preferredmeasurement range for different parts of the object along the nominalmeasurement line.
 8. A method as claimed in claim 6, in which the formof the route the preferred measuring range takes relative to the surfaceis substantially the same for successive traverses, but in which heightof the route from the surface at at least one point along the nominalmeasurement line is different for different traverses.
 9. A method asclaimed in claim 6, in which surface measurement data obtained withinthe preferred measuring range from different traverses is collated so asto form a measurement data set representing the surface of the objectalong the nominal measurement line.
 10. A method as claimed in claim 1,comprising generating and executing, as part of a second scanningoperation, a new course of relative movement of the analogue probe andobject.
 11. A method as claimed in claim 10, in which the new course ofrelative movement comprises the contact analogue probe traversingsubstantially the same line of measurement across the surface of theobject, but in which the relative movement is controlled such thecontact analogue probe obtains measurements within its preferredmeasurement range for a greater proportion of the course of movementthan for that of the scanning operation.
 12. A method as claimed inclaim 1, in which the contact analogue probe's preferred measurementrange is a preferred range of deflection of the contact analogue probe.13. A computer program comprising instructions which when executed by amachine tool apparatus causes the machine tool apparatus to perform themethod of claim
 1. 14. A computer readable medium comprisinginstructions which when executed by a machine tool apparatus causes themachine tool apparatus to perform the method of claim
 1. 15. A machinetool apparatus comprising a machine tool, a contact analogue probehaving a deflectable stylus mounted on the machine tool, and acontroller configured to control the relative movement of the contactanalogue probe and an object to be measured so as to so as to collectscanned measurement data along a nominal measurement line on the surfaceof the object, and in particular so as to control the contact analogueprobe and/or object in accordance with a course of relative motion suchthat the position of the preferred measuring range relative to thesurface of the object is controlled in a manner that, based on assumedproperties of the surface of the object, will cause the contact analogueprobe to obtain data within its preferred measuring range, as well as toexceed its preferred measuring range, along the nominal measurement lineon the surface of the object.
 16. A method of scanning an object usingan analogue probe mounted on a machine tool, the analogue probe having apreferred measurement range, the method comprising performing a scanningmeasurement operation which comprises moving the object and analogueprobe relative to each other so that the analogue probe obtains scannedmeasurement data along a nominal measurement line on the surface of theobject, in which some of the data obtained during the scanningmeasurement operation along the nominal measurement line is within theanalogue probe's preferred measurement range and some is outside theprobe's preferred measurement range.